change Module::define_function to take TrapSink instances

Experience with the `define_function` API has shown that returning
borrowed slices of `TrapSite` is not ideal: the returned slice
represents a borrow on the entire `Module`, which makes calling back
into methods taking `&mut self` a bit tricky.

To eliminate the problem, let's require the callers of `define_function`
to provide `TrapSink` instances.  This style of API enables them to
control when and how traps are collected, and makes the `object` and
`faerie` backends simpler/more efficient by not having to worry about
trap collection.

cranelift/object/src/lib.rs

@@ -26,10 +26,8 @@
 )]
 
 mod backend;
-mod traps;
 
-pub use crate::backend::{ObjectBackend, ObjectBuilder, ObjectProduct, ObjectTrapCollection};
-pub use crate::traps::ObjectTrapSink;
+pub use crate::backend::{ObjectBackend, ObjectBuilder, ObjectProduct};
 
 /// Version number of this crate.
 pub const VERSION: &str = env!("CARGO_PKG_VERSION");